Sharp chance? (10)
I believe the answer is:
accidental
I believe this clue is a double definition.
'sharp' is the first definition.
(in music, sharps and flats are accidentals)
'chance?' is the second definition.
('accidental' can be a synonym of 'chance')
(Other definitions for accidental that I've seen before include "Unintended - sort of note" , "Chance, inadvertent" , "not intended by composer?" , "Casual" , "change of key" .)